1. Field of the Invention
The present invention relates to wire-shaped electrodes which are used for machining workpieces by electro-erosion.
2. Description of the Prior Art
In such machining, described for example in the document FR-A-2 418 699, a wire-shaped electrode is driven along a path close to a metal workpiece to be machined. An electric generator produces a potential difference between the metal workpiece to be machined and the metal wire forming the electrode. A spark is produced in the machining zone between the wire forming the electrode and the metal workpiece, and progressively erodes the workpiece.
The rate of eroding the mechanical workpiece which can be obtained depends on a large number of parameters, particularly on the electric generator, the sparking distance and the machining fluid present in the sparking zone. However, it has been discovered that the rate of erosion also depends on the nature of the materials forming the wire-shaped electrode.
The document CH-A-646 083 mentions the possibility of forming a wire electrode for spark erosion by providing a good conducting core, made from steel or copper, coated with a zinc base alloy layer. The presence of zinc limits the surface temperature of the electrode during sparking, because zinc has a relatively low evaporation temperature. The result is less heating of the core in the sparking zone, which reduces the risks of breakage.
The document CH-A-646 083 also teaches the introduction, in the coating containing zinc, an amount of nickel whose effect is to increase the electric resistivity of the coating and so reduce its thermal conductivity. The result is that the coating thermally isolates the metal core in the sparking zone and reduces heating thereof, which also reduces the risks of breakage.
These known techniques, tending to reduce the risks of breakage, are however not sufficient for electro-erosion wires whose diameter is small, for example less than 0.30 mm. For small diameters, it has in fact been discovered that the tendency to breakage of the wires increases substantially apparently more than the reduction of diameter.
Now modern electro-erosion techniques, particularly for obtaining high machining precision, require the use of filiform electrodes whose diameter is the smallest possible.
From these observations, the present invention proposes forming a filiform electrode for electro-erosion with which the risks of breakage are substantially reduced, substantially increasing the rate and precision of erosion of a metal workpiece, the other electro-erosion parameters being kept constant.
The problem which the invention attempts to solve is to increase the electric conductivity of the core coated with a metal coating, preferably a coating containing a proportion of zinc, so as to reduce heating of the core due to the passage of the electric current in the core as far as the sparking zone.
It is in fact apparent that, with the filiform electrodes obtained by the methods described in the above documents, the electric conductivity of the core is deteriorated during formation of the coating, and such deterioration is all the greater when the core has a relatively small diameter, i.e. when the electrode has a diameter less than 0.30 mm.
The invention resides particularly in the fact of having demonstrated that the breakage of electrodes with a metal core covered with a zinc based coating obtained by known processes, result from the degradation of the electric conductivity of the core during formation of the coating.